Techniques for liquefying biomass at a normal pressure has advantages of mild reacting condition requirements, simple equipment requirements, and is capable of replacing petrochemical compounds and fuel oils when made into appropriate products, etc. Thus, currently there is much research on developing techniques for liquefying biomass at a normal pressure. The most typical technique for liquefying biomass at a normal pressure, adopts wood powder and phenol as a liquefying solvent for a process, at a ratio of 1:8, and 3% of H2SO4 as a catalyst. The reacting temperature and time used is 180° C. and 2 hours, respectively. The residue rate of the liquefied product obtained from the process is 30% (Journal of Beijing Forestry University, Vol. 26, issue 5, 2004, p. 67). In another example, a process adopts wood powder and a polyhydric alcohol (polyol) as a liquefying solvent, such as ethylene glycol for a process, at a ratio of 1:4, and 0.25% of H2SO4 as a catalyst. The reacting temperature and time used is 250° C. and 1 hour, respectively. The residue rate of the liquefied product obtained from the process is 5% (Rezzoug et al., 2002). However, because the processes use a corrodent catalyst and the reacted phenol or ethylene glycol is hard to be recycled, the process has not been widely commercially applied.
In 2002, it was disclosed that an alkyllimidazolium chloride ionic liquid is capable of dissolving cellulose at a low temperature (Swatloski et al., 2002). Furthermore, Zhang (Zhang et al., 2010) discovered that the water in an ionic liquid at mild conditions had a high Kw value (which was higher than pure water at room temperature conditions for 3 orders of a magnitude). The disclosure was very important for biomass hydrolysis and liquefaction since such a high Kw value typically is only able to be reached at an extremely high temperature or sub-critical conditions.
U.S. Pat. No. 7,959,765 B2 discloses using an alkyllimidazolium chloride ionic liquid to partially dissolve wood powder to promote reactions for liquefaction and depolymerization. The method comprises mixing pine wood powder with [AMIM]Cl ionic liquid to form a mixture with a concentration of 10 wt %, placing the mixture into a flask with an opening connected to a vacuum distillation apparatus and slowly raising a temperature of the mixture to 200° C., and reacting the mixture for 20 minutes. The liquid-oil product obtained from 20 minutes of vacuum distillation had a 15% residue rate. WO2011028776 discloses a mixture of hydrates of ZnCl2, CaCl2, etc. which forms an ionic liquid at a temperature of 200-600° C., which partially dissolves wood powder to promote a reaction for liquefaction and depolymerization. In the process, a catalyst may be used to perform reactions such as hydrogenation, hydrogenolytic cleavage, catalytic cracking and thermal cleavage to raise the quality level of pyrolysis oils.
Accordingly, it is known that for liquefying and depolymerizing biomass, effectively, using ionic liquid as a solvent is an important technique. Based on the articles “Electrochemical aspects of Ionic liquids” written by Hiroyuki Ohno, and “Ionic liquids in synthesis” written by Peter Wasserscheid and Tom Welton, or “AIChE Journal, Vol. 47, 2001, p. 2384-2389” issued by Joan F. Brennecke and Edward J. Maginn, it is known that an ionic liquid is defined as “a liquid that consists entirely of ions with a melting point of less than 100° C.”. A cation structure of a common ionic liquid mainly is pyridinium, imidazolium or pyrrolidinium. However, ionic liquid is expensive and thus, is not mass produced. Accordingly, it is not convenient to purchase ionic liquid commercially.
Therefore, a new method for liquefying biomass, which has the advantages of having excellent liquefying effects with low costs is needed.